Fluorine-containing polymers have been widely used as electrically insulating materials owing to the fact that they possess excellent electrical characteristics, such as typically having a low dielectric constant as compared to other polymers and having a small dielectric loss tangent. Moreover, foaming of the polymers has been studied in order to further improve electrical characteristics.
Numerous methods have been proposed for obtaining fluorine-containing polymer foams. Known examples include a method wherein organic or inorganic chemicals are added to a polymer and foaming is brought about by gases. Primarily inert gases such as N.sub.2 and CO.sub.2 or volatile organic liquids such as hydrogen chloride are blown under high pressure in gas or liquid form into a polymer which has been melted in a molding machine and foamed by expanding the gases contained within the polymer when the melted polymer emerges from the molding machine. A method wherein a polymer is impregnated with a suitable organic solvent and foamed by forming the impregnated polymer at a temperature equal to or higher than the boiling point of the organic solvent is also used.
The applicants have in the past studied hollow microspheres with a low dielectric constant, such as hollow microspheres made of glass, and have proposed that the microspheres be mixed into a tetrafluoroethylene resin or another fluorine-containing polymer which can be melt extruded. However, with the insulating materials described above which ave hollow microspheres mixed in, because rolling treatments are performed and the materials are subject to external forces such as compression, the hollow microspheres are easily broken. Therefore, the problem exists that electrical characteristics will fail to be enhanced even if a large number of microspheres are packed into the polymer. Other problems remain to be solved, such as the fact that the manufacture of the above materials is difficult and the products obtained are essentially limited to sheet form.
Further, in the above material which is formed in an extrusion molding device the hollow microspheres are easily broken by screw pressure in the extrusion molding device. Also adding to the fact that the melt viscosity of the filled material rises precipitously and the fluidity decreases if a large number of microspheres are packed into the polymer, the upper limit to the amount of microspheres which can be blended in remains at no more than about 10 wt %. Improvement of the electrical characteristics is thus limited.
Known fluorine-containing polymers that have been used conventionally as electrically insulating materials include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether copolymers (PFA), poly(chlorotrifluoroethylene) (PCTFE), ethylene-tetrafluoroethylene copolymers (ETFE), and ethylene-chlorotrifluoroethylene copolymer (ECTFE). These fluorine-containing polymers are chemically stable and in general are insoluble in solvents, and hence useful molding processes for them have been compression molding and the combined use of melt molding with machining. However, because external forces such as compressive forces and shearing forces are necessarily applied to the molded material in these molding methods, when hollow microspheres of glass are packed in to the above-mentioned fluorine-containing polymers, it has not been possible to escape the breakage of the glass microspheres.